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Small cylinders of a rare element, placed in the centre of a huge volume of very cold Argon in liquid state, which in turn is surrounded by a water tank, in the rooms of the largest astroparticle laboratory in the world, that of the Italian Istituto Nazionale di Fisica Nucleare under a 1400-metre high mountain in the heart of Italy, in the Gran Sasso area.
This is the GERDA (Germanium Detector Array) experiment which will be inaugurated on the 9th of November 2010, in search of a spontaneous decay of matter which is so rare that it is more arduous to find than a needle in a haystack: in physics it is referred to as double beta decay without neutrino emission. In order to make sure that it occurs, the neutrino needs to coincide with its antimatter particle; the decay, however, is so rare that it requires long, careful and very delicate observation. It is like perceiving a single specific note during a concert season: you need a room with perfect acoustics to hear it.
The same applies for the GERDA experiment. Its “acoustics” are guaranteed by the liquid Argon, water and rock volumes, placed in a nesting-doll structure, which protect the typical “note” of the experiment from billions of particles arriving from the Universe recesses, but also from the Gran Sasso rocks. These disturbance “sounds” are blocked out by the rock above the laboratory (the former), and finally by the “nesting dolls” which protect the experiment (the latter).
Double beta decay without neutrino emission is a small very rare note deriving from the matter, which is important for scientist because, if proved, it would confirm that neutrino particles are so strange that they coincide with their antimatter particles (Majorana neutrino). This would provide crucial information in terms of sub-nuclear physics, astrophysics and cosmology.

The structure of GERDA

GERDA is an international collaboration involving fifteen institutes from Italy, Germany, Russia, Switzerland, Poland and Belgium. Initially the experiment will be conducted using 8 detectors the size of a tine can and weighing two kilograms each. They are made of hyperpure germanium monocrystals, a semiconductor, enriched with the isotope germanium-76. The nuclei of the crystal decay and the particles, emitted (electrons) during the double beta decay of the germanium-76 nuclei, release their energy in the form of a “trail” in the detector. In GERDA the detectors are used, at the same time, to “generate” and to reveal the particles emitted during the decay.
The GERDA crystals are suspended in a tank (cryostat) which is six meters tall and four meters wide, containing liquid argon. The cryostat, in turn, is placed inside a ten-meter high water tank with a ten-meter diameter, which serves as further screen.
After an initial period, other detectors will be commissioned.

The physics of GERDA

Together with photons, neutrinos are the most widespread particles in the Universe. They are, however, particularly elusive because they interact with matter only weakly.
According to some models, neutrinos coincide with their anti-particle; if this original property were true, it would prove some important elementary particle physics theories and it would expand our knowledge concerning the structure of matter.

The GERDA experiment is aimed at testing these models by searching for the very rare double beta decay without neutrino emission; this means that two of the nucleus neutrons are transformed (decay) into two protons, two electrons and two neutrinos. However, the two emitted neutrinos cancel one another and therefore do not emerge from the nucleus.
The observation of the double beta decay without neutrino emission would serve to directly measure the mass of the electronic neutrino; the value of the latter has a great impact on Universe development models, especially as regards the formation of galaxy masses.